Crystal Radio

  
The source connection is tapped into the tuning coil low down, as a means of impedance matching with the headphones and acts as the anode, while the drain connects with your headphones as the cathode, to complete the detector part of the circuit. According to the experts on the net, much success has been enjoyed by one and all, and there's a lot of chatter about this most recent innovation in the realm of simple AM receivers (292 posts so far, to the 'Rap'n'tap' chatroom of the American Crystal Set Society alone!- www.midnightscience.com). One of the most curious aspects of this little beauty is that the antenna/ground system I employ basically entails a 'short' antenna, and a water pipe ground. Ideally, short antenna wires work best near the top of the tuning coil, but in this case, the best position seems to be right at the bottom tap! Normally, this would send all my weak locals into a spin, and shift the whole band up towards the top end of the tuning cap's range.
Crystal Radio - schematic

What is presented in this article, is my version of the QST arrangement, and I'm using the humble 2N5484 JFET, purchased from JAYCAR Electronics for around $2.00. Yes, a JFET, not a MOSFET! Why? Well, my basic knowledge of FET devices at the time of acquiring the QST design was a little rusty, to say the least, not having experimented with them for some time, so after a bit of scratching around on the net, and down at the local library, I reclued myself as to their peculiarities. Devices that are static sensitive have protection diodes on the inputs, and the ALD device certainly has those. Basic JFETs also have a protective diode between the gate and source connections, presumably for the same reason, but neither the QST design, nor my adaptation of it, use any internal diodes as a rectifier. You can experiment with that as a start - simply replace your germanium or Schottky diode with the gate and source of a JFET device, and you'll get reception all right - but it does seem a bit mushy or scratchy, somewhat like a poor Schottky diode that distorts on low signal levels (BAT46's come to mind?) You can still obtain reasonable results by tapping the gate further up the tuning coil, if you want to. This version of the receiver will work quite OK with air cored inductors, providing you use PVC insulated wire. I don't recommend enamelled copper wire, as it can be down a bit on sensitivity and selectivity. Get some good quality,...



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